This proposal is directed toward an understanding of the physical and chemical events which govern and accompany the expression of genes that specify ribosomal RNA in eukaryotes. The program described will focus primarily on these points and in that order of priority: 1) Determination of the capacity of isolated rat nuclei and nucleoli for sustained, ribosomal RNA synthesis. 2) The dissection and reconstitution of isolated nucleoli capable of sustained rRNA synthesis. 3) A search for nucleolar proteins possibly involved in the regulation of rRNA synthesis. 4) A search for amphibian mutants abnormal in rRNA synthesis and abnormal in the organization of the nucleolus. 5) Structural studies of the rat rRNA species and of their precursors.